Decontamination methods are used in a broad range of applications, and have used an equally broad range of decontaminating agents. As used herein the term “decontamination” refers to the inactivation of bio-contamination, and includes, but is not limited to, sterilization and disinfection.
During a decontamination cycle of a typical hydrogen peroxide vapor decontamination system, an aqueous solution of hydrogen peroxide (e.g., about 30% to 59% hydrogen peroxide by weight) is injected into a vaporizer. The vaporizer vaporizes the aqueous solution of hydrogen peroxide, thereby generating a hydrogen peroxide vapor that is carried into a decontamination chamber by a carrier gas (e.g., air).
Gaseous and vaporous decontamination systems rely on maintaining certain process parameters in order to achieve a target decontamination assurance level. For hydrogen peroxide vapor decontamination systems, those parameters include, but are not limited to, the concentration of the hydrogen peroxide vapor, the degree of saturation, the temperature and pressure, and the exposure time. By controlling these parameters, the desired decontamination assurance levels can be successfully obtained while avoiding condensation of the hydrogen peroxide due to vapor saturation. In this regard, condensation of hydrogen peroxide is ordinarily not desired, since it can increase aeration time, cause corrosion, and lead to hazardous conditions. Some studies have also shown that condensation of the hydrogen peroxide may also inhibit the effectiveness of the hydrogen peroxide vapor.
Considering only temperature, condensation of hydrogen peroxide vapor occurs when an actual concentration of hydrogen peroxide vapor exceeds a saturation concentration of hydrogen peroxide vapor (also referred to herein as a “dew point” concentration) for a given temperature. In order to avoid condensation of the hydrogen peroxide vapor during a decontamination cycle, care must be taken to insure that the actual concentration of the hydrogen peroxide vapor in the decontamination chamber does not exceed the saturation concentration for the temperature in the decontamination chamber.
As previously indicated, atmospheres of hydrogen peroxide vapor typically include water vapor. The concentration of water vapor found in a vaporized hydrogen peroxide atmosphere depends on the initial concentration of water in the aqueous hydrogen peroxide-water mix and the degradation of vaporized hydrogen peroxide into water vapor during a decontamination cycle. In addition to showing a temperature dependency, the saturation concentration of hydrogen peroxide vapor is also a function of water vapor concentration. For example, it is observed in a vaporized hydrogen peroxide/water vapor atmosphere that the higher the actual concentration of water vapor, the lower the saturation concentration of hydrogen peroxide vapor.
The amount of vaporized hydrogen peroxide that can be produced per unit time (i.e., the injection rate) is limited by the capacity of the vaporizer. Therefore, in smaller enclosed areas, higher concentrations of hydrogen peroxide may be easily attained using the maximum injection rate. In larger enclosed areas (e.g., rooms), it may only be possible to obtain lower concentrations of hydrogen peroxide. As the concentration of hydrogen peroxide decreases, the time required to inactivate biocontamination increases exponentially.
Typically, a D-value is used to express the time (i.e., “decimal reduction time”) required for a one log reduction of bioburden (i.e., a reduction in the viable microbial population by 90%). Accordingly, xD expresses the time required for x log reduction of bioburden. For example, to obtain a “kill” of 6 log reduction of Bacillus (Geobacillus) stearothermophilus using STERIS® VAPROX® Hydrogen Peroxide Sterilant, the object being decontaminated must be exposed to the STERIS® VAPROX® Hydrogen Peroxide Sterilant at a concentration of 250 ppm for an exposure time of 1.5 hours, or at a concentration of 400 ppm for an exposure time of 0.5 hours. However, once a decontamination cycle has commenced, two possible conditions may exist that prevent the use of the higher concentration (e.g., 400 ppm in the case of STERIS® VAPROX® Hydrogen Peroxide Sterilant) for the shorter exposure time. These two conditions are: (1) a hydrogen peroxide concentration that exceeds the dew point concentration, or (2) an inability to obtain the higher concentration level (e.g., 400 ppm) within the enclosure (e.g., room) due to vaporizer capacity limits (i.e., an insufficient maximum injection rate).
Existing decontamination systems lack control means for determining whether a condensation condition exists, and for determining an optimal concentration level from among of a plurality of possible concentration levels.
The present invention addresses these and other problems, and provides a decontamination system that includes feedback control to monitor condensation conditions and to determine an optimal concentration level.